Is There Bone Marrow in Fish Bones?

December 18, 2025 •

3 min read

Most bony fish do not have a functional bone marrow, a stark contrast to mammals where marrow is the primary site of blood cell production. Instead, fish rely on other, specialized organs to perform this vital function, a key evolutionary difference in their physiology.

Quick Summary

This article explores the question of whether fish bones contain marrow and reveals the surprising places fish produce blood. It details the anatomical differences between bony fish and mammals, and where fish hematopoiesis takes place.

Key Points

No Marrow in Most Fish: The majority of modern bony fish lack true bone marrow for producing blood cells.
Head Kidney is Key: The primary blood-forming organ (hematopoietic organ) in most bony fish is the head kidney.
Other Organs Help: The spleen also acts as an accessory site for blood production and a reserve for red blood cells.
Evolutionary Difference: This is a major evolutionary distinction from mammals, which produce blood primarily in bone marrow.
Nutritional Value: Despite lacking marrow, fish bones are rich in calcium and phosphorus, making them valuable for supplements and fortifying food.
Evidence in Fossils: Ancient fossil fish show evidence of marrow-like tissue, indicating the ancestral capacity existed before modern fish adapted their methods.

Fish Hematopoiesis: A Different Approach to Blood Production

Unlike mammals, the vast majority of bony fish do not produce blood cells within their bones. This is a fundamental divergence in vertebrate anatomy tied to evolutionary adaptations for life in an aquatic environment. While the bones of bony fish provide structural support, protection, and are composed of calcium phosphate, they are generally lighter and more flexible than mammalian bones. The primary site for blood cell formation, or hematopoiesis, in most teleost (bony) fish shifts to a different set of organs entirely.

The Head Kidney: The Fish's Bone Marrow Equivalent

For most teleost fish, the head kidney (or pronephros) is the central hematopoietic organ, performing the function that bone marrow does in mammals. This specialized tissue in the anterior part of the kidney is rich in hematopoietic stem cells that generate red blood cells, white blood cells, and thrombocytes throughout the fish's life. The head kidney contains a matrix of vascularized reticuloendothelial tissue, free macrophages, and melanomacrophage centers that all support the blood-forming process. In fact, the cellular composition of the teleost head kidney is remarkably similar to mammalian bone marrow, despite its location.

Other Hematopoietic Organs in Fish

While the head kidney is primary, other organs also play a role in blood cell production and storage for fish.

Spleen: In many teleosts, the spleen acts as an accessory hematopoietic organ and is also a significant reservoir for red blood cells.
Thymus: The thymus is also a paired organ in fish, located under the operculum (gill cover), and is involved in the production of T-lymphocytes for the immune system.
Intestinal Submucosa: The digestive tract's submucosa can also contain lymphoid tissue contributing to the immune system and blood cell production.

Do any fish have bone marrow?

While the general rule is no, some exceptions and evolutionary insights exist. Some ancient or specific species of bony fish have shown evidence of marrow-like tissue. Research on 380-million-year-old fossil fish revealed marrow tissue in their fin bones, indicating its evolutionary precedence in vertebrates. Some modern bony fishes, like the bowfin, also possess a mature, though extraskeletal, bone marrow organ that extends into the cavities of the skull. This suggests that the capacity for bone marrow existed in fish ancestors, but the function was largely delegated to other organs in modern species.

Comparison: Fish vs. Mammal Blood Production


Feature	Bony Fish (Teleosts)	Mammals
Primary Hematopoietic Site	Head Kidney (Pronephros)	Bone Marrow
Secondary Hematopoietic Organs	Spleen, Thymus, Intestinal submucosa	Spleen (stores RBCs), Lymph nodes, Liver (fetal)
Erythrocyte Nucleus	Retain their cell nucleus throughout their lifespan	Lose their nucleus upon maturation
Thrombocyte Characteristics	Remain nucleated and are involved in phagocytosis	Cell fragments (platelets), lack a nucleus
Blood Cell Maturation	Many immature cells released into circulation for final maturation	Occurs almost exclusively within the bone marrow

The Role of Minerals in Fish Bones

Despite lacking traditional marrow, fish bones are still valuable. They are rich in minerals like calcium and phosphorus, primarily in the form of hydroxyapatite. This makes them a potential, highly bioavailable source of calcium for dietary supplements and fortified foods. Research shows that processed fish bone powder can have better calcium bioavailability than traditional calcium carbonate supplements. Fish bones also contain collagen and protein, which can be extracted for various non-food products like gelatin and adhesives.

Conclusion

In summary, the answer to whether there is bone marrow in fish bones is generally no for most modern bony fish. The evolutionary divergence of aquatic and terrestrial vertebrates has led to a different physiological strategy for blood production. While a mammal’s circulatory system depends heavily on the marrow inside its bones, a fish's system relies on the specialized head kidney to produce and replenish its blood cells. This adaptation highlights the remarkable ways life adapts to its environment, proving that vertebrates have found multiple successful ways to perform this fundamental biological process.

Learn More About Fish Anatomy

For further reading on the complex and fascinating world of fish anatomy and physiology, the National Institutes of Health (NIH) provides access to a wealth of scientific literature. Their site is an excellent resource for anyone interested in the biological differences between aquatic and terrestrial vertebrates.

Frequently Asked Questions

Fish primarily produce blood cells in a specialized organ called the head kidney, located in the anterior part of their body. The spleen also serves as a hematopoietic organ and a reservoir for blood cells.

Yes, fish bones are structurally different from human bones. They are generally lighter and more flexible, adapted for an aquatic environment. They also lack the central medullary cavity filled with bone marrow found in most terrestrial vertebrates.

No, cartilaginous fish such as sharks, skates, and rays do not have bones or bone marrow. Their skeleton is composed of cartilage. Instead, they use organs like the spleen and a specialized organ called the Leydig's organ for blood production.

Fish bones are primarily composed of minerals like calcium phosphate, which give them their hard structure. They also contain protein, including collagen.

Yes, fish bones are a rich, bioavailable source of calcium. They can be processed into supplements or added to fortified foods to help increase calcium intake.

No. While mammals and many other terrestrial vertebrates produce blood in their bone marrow, most fish and some amphibians use other organs like the kidney and spleen for this function.

The bowfin is a rare example of a bony fish that possesses a mature bone marrow organ, although it is considered an evolutionary outlier among modern bony fish. Fossil evidence also suggests marrow-like tissue in some ancient fish.